The variable optical attenuator (VOA), which attenuates light injected from an optical transmission channel for input (typically, optical fiber) and outputs it to an optical transmission channel for output (typically, optical fiber), can variably adjust the light attenuation. As a method for controlling the light attenuation in such a variable optical attenuator, various types have been proposed. For example, there are a mechanical type in which a shutter is inserted/removed halfway an optical path between opposed end faces of optical fibers and the light attenuation is adjusted using a shading level by the shutter, and a type in which an optical element such as Faraday rotator or thermooptic element is disposed halfway the optical channel.
However, in the first variable optical attenuator of the mechanical type, there is a problem of wavelength dependence or polarization-dependant loss because of diffraction at an edge of the shutter. Furthermore, a conventional actuator used in the mechanical-type attenuator is large in size, therefore miniaturization of the variable optical attenuator has been difficult.
In the second variable optical attenuator using the optical element, the variable optical attenuator is expensive because the expensive optical element such as Faraday rotator or thermooptic element is required, in addition, since it does not have self-holding capability of the light attenuation, current needs to be continuously applied to an electrical element for affecting on the optical element, therefore power consumption has been large. Also, an electrical element for affecting on other optical elements or an optical element is necessary with regard to the optical element, therefore a structure of the attenuator has been apt to be complicated.
As a variable optical attenuator using a light reflection surface, an attenuator disclosed in U.S. Pat. No. 6,137,941 is known. FIG. 1 is a schematic view showing a structure of the conventional variable optical attenuator. In the variable optical attenuator, as shown in FIG. 1, a lens 3 is disposed on end faces of an optical transmission channel for input 1 and an optical transmission channel for output 2 arranged parallel, a mirror 4 is provided at a position distant from the lens 3 only by the focal distance of the lens f, and the mirror 4 is rotatably supported by a fulcrum 5. Here, an intermediate line between the optical transmission channel for input 1 and the optical transmission channel for output 2 coincides with an optical axis of the lens 3, and the fulcrum 5 is located on an extension of the line. A piezoelectric actuator 7 is inserted between the mirror 4 and a base 6, and the piezoelectric actuator 7 is expanded and contracted with being controlled by a controller 8, thereby tilt of the mirror 4 can be optionally adjusted.
Thus, when the mirror 4 is perpendicular to the optical axis of the lens 3, light emitted parallel to the optical axis of the lens 3 from the optical transmission channel for input 1 refracts when it transmits through the lens 3 and then reaches the mirror 4, and the light reflected on the mirror 4 refracts when it transmits through the lens 3 and becomes parallel to the optical axis of the lens 3 and then it is injected into the optical transmission channel for output 2. In this case, when an optical axis of the light injected into the optical transmission channel for output 2 coincides with an axis center of the optical transmission channel for output 2, a quantity of light injected into the optical transmission channel for output 2 is maximized (the light attenuation is minimized). On the contrary, when the mirror 4 is tilted by the piezoelectric actuator 7, the optical axis of the light that is emitted from the optical transmission channel for input 1 and reflected on the mirror 4 and then returned to the optical transmission channel for output 2 is displaced from the axis center of the optical transmission channel for output 2, and the quantity of light injected into the optical transmission channel for output 2 decreases, therefore as the tilt of the mirror 4 increases, the attenuation of the light injected into the optical transmission channel for output 2 increases.
According to the variable optical attenuator having such a structure, the problem such as the wavelength dependence in the variable optical attenuator of the shutter type does not occur, in addition, the problem of high price due to the optical element can be avoided.
However, in the variable optical attenuator having such a structure, the lens 3 must be distant from the mirror 4 only by the focal distance of the lens 3, in addition, to reduce aberration of the light emitted from the optical transmission channel for input 1 or the light injected into the optical transmission channel for output 2, a portion near the optical axis of the lens 3 needs to be used as much as possible, and a short-focus lens can not be used, therefore the miniaturization of the variable optical attenuator has been restricted in such a structure. In the method of tilting the mirror 3, since the optical axis of light injected into the optical transmission channel for output 2 sensitively displaces even upon slight tilt of the mirror 3, the tilt of the mirror 3 needs to be controlled severely, therefore accurate control of the light attenuation has been difficult. Since the piezoelectric actuator is also used in this variable optical attenuator, the mirror 3 can not hold its angle by itself, resulting in large power consumption.